1. Field of the Invention
This invention relates to electrochemical cells containing liquid sodium as the anodic material.
2. Prior Art
In sodium-sulphur cells, liquid sodium forming the anodic material is separated by a solid electrolyte, usually beta-alumina, from sulphur/sodium polysulphides constituting the cathodic reactant. The anodic and cathodic materials, if allowed to mix, as would occur if the electrolyte was breached, react exothermically. It is therefore the practice in such cells to maintain only a small quantity of sodium over the anodic surface of the electrolyte element, gas pressure and/or a capillary or wicking system being used to force or draw up sodium from a reservoir into the anodic region adjacent the electrolyte surface. In a tubular cell, the anodic region is an annular region between one surface of the electrolyte tube and an anodic current collector; the sodium may be inside or outside the electrolyte tube but conveniently the anode current collector is a metal housing surrounding the electrolyte tube. To reduce the quantity of sodium in this annular region, it is a common practice to fill this region with carbon powder or small glass beads or alumina, these materials being chemically and electrochemically inert with respect to the sodium and the electrolyte material. Reference may be made to U.S. Pat. No. 4,048,391 which refers to the possible use of glass beads, steel shot, ceramic spheres or powder, silica carbide granules and graphite powder or spheres.
These materials used so far have various drawbacks. The desirable requirements for such a filler are that
(a) it should be wetted by sodium at 150.degree. C., PA0 (b) it should not outgas at the filling or operating temperatures, PA0 (c) it should be resistant to corrosion by liquid sodium, PA0 (d) it should have a high packing density, PA0 (e) it should not contaminate the sodium electrode, PA0 (f) it should have a particle size distribution which will adequately fill the anodic region without individual particles bridging between the electrolyte and the current collector or housing, PA0 (g) it should not react exothermically with sulphur or sodium polysulphides, and PA0 (h) it should have a high heat capacity per unit volume. As stated above, graphite or carbon particles or glass spheres have been most commonly employed heretofore but outgassing problems occur with both these materials. With graphite or carbon particles, poor wetting by liquid sodium at 150.degree. C. has been found such as to cause problems in filling the cell with sodium.